[OSGeo Africa] Digitising a 1929 SG diagram

Patrick Ric - Hansen patrick at compuplot.com
Sun Jan 4 23:40:31 PST 2015

Hi Hanlie,

This is probably a local system. You must capture it with these co-ordinates and then transform it onto Lo. WGS using common points. You will have to look for some survey records to do this.


Patrick Ric-Hansen
Professional GIS Practitioner 
Phone:     27+31+767 1327
Fax':         27+31+767 4691
Mobile:    27+82 442 3077

-----Original Message-----
From: africa-bounces at lists.osgeo.org [mailto:africa-bounces at lists.osgeo.org] On Behalf Of Hanlie Pretorius
Sent: 05 January 2015 09:05 AM
To: Africa local chapter discussions
Subject: [OSGeo Africa] Digitising a 1929 SG diagram


I have a problem digitising a 1929 SG diagram, which I have attached.

I'm not sure what the units are, but I assumed they were Cape Feet (conversion factor = 0.314855575). As you can see, the coordinates are not in the range that one would expect, so added the following constants that I got off a 1936 diagram of the same area to the Cape Feet values:

Constant for Survey Y 	24000
Constant for Survey X 	9972000

However, nothing I do can place the points in the correct position. I have tried the QGIS Cape /Lo21 (EPSG 22281) coordinate system and not swapping X and Y or multiplying by minus 1.

I have also tried the QGIS South African CRS CAPE_NO_21
(ZANGI:ZANGI:CPNO21) coordinate systems, and swapping X and Y and multiplying by minus 1

The diagram is not oriented correctly or it lands in the wrong spatial location.

I also tried a Cape Rds conversion, but the coordinates are still in the incorrect range and seems to need some kind of constant added to them.

Can anyone perhaps take a look and suggest a work flow that should put it in the correct location? I want to get the points in the coordinate table in GIS format so that I can georeference the diagram.


Can someone perhaps take a look and

More information about the Africa mailing list